The present invention relates to a fingerprint detecting device and, more particularly, to a fingerprint detecting device which irradiates light on the skin surface of a finger through a transparent body and detects a fingerprint pattern from reflected light.
Generally, as a scheme of irradiating light on the skin surface of a finger pressed against a transparent body and detecting a fingerprint pattern from reflected light, a reflection light scheme using reflection of light on the transparent body surface has been proposed, as disclosed in Japanese Patent Laid-Open No. 1-145785.
FIG. 3 schematically shows a conventional fingerprint detecting device based on the reflection light scheme. Referring to FIG. 3, reference numeral 1 denotes a transparent body such as an optical glass member having a triangular prism shape; 1S, a surface of the transparent body 1; 3, a finger placed on the surface 1S of the transparent body 1 and having a skin surface 3A with ridge/valley portions; 4, a light source; and 5, an image detection unit. Light from the light source 4 is irradiated on the skin surface 3A through the transparent body 1, and reflected light is detected by the image detection unit 5.
In this case, the relationship in refractive index between the transparent body 1, the skin surface 3A, and air, and the angle of light incident on the surface 1S of the transparent body 1 are appropriately selected. Normally, the ridge portions (projecting portions) of the skin surface 3A contact the transparent body 1 via sweat or grease adhered on the surface of the finger. The refractive index of sweat or grease is higher than a refractive index na of air and close to a refractive index nb of the transparent body 1 such as an optical glass.
For this reason, reflection occurs only on the surface 1S of the transparent body 1 contacting the valley portions (recessed portions) of the skin surface 3A, where air is present. On the other hand, the reflection does not take place at the ridge portions of the skin surface 3A which directly contact the transparent body 1 via sweat or grease having a refractive index close to the refractive index nb of the transparent body 1, so the light transmitted through the transparent body 1 is absorbed or irregularly reflected by the finger. Therefore, the image detection unit 5 detects the fingerprint pattern of the skin surface 3A, which has bright valley portions and dark ridge portions.
As another scheme of detecting a fingerprint pattern, a scattering light scheme using scattering of light on the transparent body surface has been proposed, as disclosed in Japanese Patent Laid-Open No. 3-244092. FIG. 4 shows a conventional fingerprint detecting device using the scattering light scheme. Referring to FIG. 4, reference numeral 11 denotes a transparent body formed from parallel plates such as optical glass members; 11S, a surface of the transparent body 11; 13, a finger having a skin surface 13A with ridge/valley portions; 14, a light source; and 15, an image detection unit. Light from the light source 14 is irradiated on the skin surface 13A through the transparent body 11, and scattering light is detected by the image detection unit 15.
In this case as well, the relationship in refractive index between the transparent body 11, the skin surface 13A, and air, and the angle of light incident on the transparent body surface 11S are appropriately selected. Reflection occurs on the transparent body surface 11S contacting the valley portions (recessed portions) of the skin surface 13A, where air 18 is present, so the light does not reach the image detection unit 15.
On the other hand, the reflection does not occur at the ridge portions of the skin surface 13A which contact the transparent body 11 via sweat or grease having a refractive index close to the refractive index nb of the transparent body 11. The light passing through the transparent body 11 is absorbed by the finger 13 although the light is partially irregularly reflected and reaches the image detection unit 5. Therefore, unlike the reflection light scheme shown in FIG. 3, the image detection unit 15 detects the fingerprint pattern of the skin surface 13A, which has dark valley portions and bright ridge portions.
However, in the reflection light scheme shown in FIG. 3, the detected fingerprint pattern largely changes due to a gap between a fine three-dimensional pattern on the ridge portions of the finger 3 and the surface 1S of the transparent body 1 or the condition (dry or wet state) of the finger 3 influenced by water such as sweat, so a satisfactory fingerprint pattern cannot always be obtained.
For example, as shown in FIG. 5A, when the finger is dry, a small gap 35 is formed between a ridge portion 31 and the transparent body 1 because of the fine three-dimensional pattern on the ridge portion 31 of the skin surface 3A. Since air is present in the gap 35, light is reflected even at the ridge portions 31. On the other hand, as shown in FIG. 5B, when the finger is wet due to sweat, sweat or grease 36 is adhered between the ridge portions 31 of the skin surface 3A and the transparent body surface 1S. Light is absorbed by a portion wider than the actual ridge portion 31, so the amount of light to be reflected at a valley portion 32 is decreased.
Therefore, the ridge portion 31 of the skin surface 3A is detected not as a solid line but as a broken line depending on the condition of the finger, and more particularly, in the dry state. On the other hand, in the wet state, the ridge portions 31 and valley portions are not clearly discriminated, resulting in undesirable connection between ridge and valley pattern portions. For this reason, a satisfactory fingerprint pattern cannot always be obtained.
In the reflection light scheme shown in FIG. 3, the image detection unit 5 detects the fingerprint pattern as the obliquely photographed image data of the skin surface 3A. Distortion in this oblique image must be corrected to obtain an image viewed from the front side. In addition, since the skin surface 3A is obliquely photographed, the focal depth must be increased to obtain an appropriate in-focus position. As a result, the optical distance cannot be decreased, and size reduction is difficult.
According to the scattering light scheme shown in FIG. 4, a fingerprint pattern corresponding to the fingerprint 13A viewed from the front side can be obtained. However, since the light is irregularly reflected at the ridge portions of the fingerprint 13A, and the amount of light reaching the image detection unit 15 is small, the resultant fingerprint pattern has low contrast to the valley portions of the fingerprint 13A.
Additionally, unlike the reflection light scheme, the entire image becomes dark when no finger contacts the transparent body 11. For this reason, when a fingerprint pattern is to be detected by the image detection unit 15 using, e.g., a CCD sensor, another light source is required to adjust the white level.